ANTENNA Tuner Correct Specs But Misleading

Discussion in 'Ham Radio Discussions' started by KC3EPA, Apr 15, 2018.

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  1. W2AI

    W2AI XML Subscriber QRZ Page

    Antenna tuners are a bandage approach to matching the high antenna impedances of electrically/mechanically short HF antennas to the relatively low impedance of your transistorized finals transceiver . As previous posters pointed out; tuners DO NOT correct the mismatched impedances present and the heavy RF losses are still there.
     
  2. W0VRA

    W0VRA Ham Member QRZ Page

    Good. So I'm not crazy.

    To be more specific, I am getting lazy about building a dipole. I've got wire and ferrites for baluns and CMR and stuff, and a box to put things in.

    I looked at MFJ and they've got an antenna that's basically two of their mobile whips joined at the center as a dipole. They say (maybe I misunderstood) that I can expect a best SWR of 2:1.

    Since my rig only has about 10W to work with, I want to make sure I'm using it wisely.

    So let's ask again, sort of with an eye toward the First Law, conservation of energy/mass.

    Is a tuner dissipating RF energy that has been reflected back toward the transmitter by the antenna with an unmatched resonance?
     
  3. W0VRA

    W0VRA Ham Member QRZ Page

    Haha, let's pretend we're in high school physics where we're ignoring air drag, frictional losses, and not being in an inertial frame of reference. So, yeah, good cabling.
     
  4. KK5JY

    KK5JY Ham Member QRZ Page

    No(*).

    The tuner, if it is matched properly so as to provide a 50-ohm Z to the transmitter, actually re-reflects the power that was reflected by the antenna mismatch. So the power goes up the cable to the antenna, reflects back towards the tuner, then back to the antenna then back to the tuner, and so on. Each time the power "hits" the mismatch at the antenna, most of it is radiated, but some of it is reflected... this is true for each re-reflection at the antenna, too. So the amount of loss is mostly dependent on the cable loss, because each of those reflections down the feedline has loss. Good feedline has low loss, while cheap feedline has more loss. If you use really really really good feedline, you can run substantial SWR without much loss, even for considerable distance. If you use cheap feedline, it will burn off the power pretty fast at high SWR.

    Walter Maxwell discussed this at length in his very approachable book called "Reflections." It went through several revisions before he became SK. Every ham who does HF radio should have a copy of that book, especially the hams who work on the question pools. :)

    (*) The tuner actually does dissipate a (hopefully) very small amount of power, because the L and C elements are not perfect. They have intrinsic R, as well, and so as the power flows through them, they do have some loss. The amount of loss in them depends mostly on their Q (which implies R), and the current flowing through them. If you have a severe mismatch, especially on the low-side of 50 ohms, the increased current flow can cause increased losses in the tuner. But the short story is -- no -- the tuner is not inherently a "sink" for all that reflected power, if it is a quality tuner.
     
    NQ9L, K1FBI, AG5DB and 1 other person like this.
  5. W0VRA

    W0VRA Ham Member QRZ Page

    Is this helpful?

    upload_2018-4-16_8-57-28.png

    Oh, so it's like an exponential approaching 1, but losing a bit on every trip, and the cable loss controls how far from 1 the asymptote lies.

    Probably good to have short cables so the reflections don't have room to constructively interfere? Or at least plan on lengths so that the reflections phases stay out of sync?
     
  6. KK5JY

    KK5JY Ham Member QRZ Page

    Yes, there are usually one or two versions floating around on the Internet in PDF form. They are worth the read. Even if you don't take the time to understand the math, the general discussion he provides is very understandable to anybody with a ham license. He was a good teacher, at least in writing.
    More or less.
    It's not the interference that causes loss(*), it's how much loss is intrinsic to the cable. Just like a longer cable has more loss than a short one, a longer cable with high SWR gives each pass of the wave down the cable more length in which to burn off power than a short one. But yes, the losses add up faster in a longer cable as SWR increases than in a short cable as SWR increases.

    Think of the cable as a race track for model cars, with perfect spring-loaded bumpers at the ends. If you zing a car down the track, how many bounces can it make before the car stops moving? It depends on the length of the track, how low the friction is in the track, etc. It's an imperfect analogy, but you get the idea.

    If the match at the antenna is 1:1, then all the power from the first trip down the coax is dissipated by the antenna on the first try (which will still include ground losses, etc.). If the mismatch at the antenna is 2:1, about 1/3 of the energy is reflected by the antenna back down the feedline, and then re-reflected by the tuner back towards the antenna(**). That 1/3 of your original energy had to make the trip down the cable three times, incurring loss along the way. So it's like your cable is 3x its length for 1/3 of your power. Then, the 2:1 mismatch re-reflects 1/3 of that energy, meaning that 1/9 of your power made five trips down the feedline, and so on. At 3:1 mismatch, the reflection coefficient is closer to 50%, so half your power is reflected on the first pass, etc.

    Obviously, if the coaxial cable is lossless, it doesn't matter how many trips down the cable are made -- the energy will all eventually be dissipated by the antenna system (which will still include ground losses, etc.). When you use real cable, that cable will have losses, some number of dB per 100ft or similar. So keeping the run shorter, and using better cable, are ways to improve efficiency when running a mismatch on a feedline.

    (*) The constructive interference can cause high-voltage nodes to form along the cable, and if they are high enough, they can actually damage the cable insulation. There are also high-current nodes, that can spot-heat the cable, with similar effect. In a mobile installation, it is unlikely you can generate enough power to cause either, unless the cable is really, really, really small and cheap.

    (**) Maxwell did the math in his book to show that the wave reflected by the tuner was always in-phase with the wave coming from the transmitter, so the two add together in the direction of the antenna. For this to be true, it assumes that the tuner is properly adjusted to provide a 1:1 match between the tuner and the transmitter. The math is complex (in the "J" sense), so for people without engineering background, it looks kind of funny, but it's there for those who are interested.
     
  7. W0VRA

    W0VRA Ham Member QRZ Page

    Very good analogy.

    That's exactly what I was imagining.

    So when I'm looking for the feedline to my dipole I need to check numbers like this chart?

    upload_2018-4-16_9-55-50.png
     
  8. KK5JY

    KK5JY Ham Member QRZ Page

    If you plan to run a nonresonant dipole with a desktop tuner, then line losses are very important. If you intend to run a resonant dipole, then losses are important, but less critical. Voltage ratings are also important if you intend to run power, because you need to plan for the voltage on the line to be that of a matched line at the power level you want to run, multiplied by at least the mismatch ratio.

    What Maxwell suggested was to run a dipole that was resonant somewhere in-band, but then use a desktop tuner to essentially "trim" the SWR, which should be no worse than perhaps 3:1 in the case of 80m band edges. Then he suggested a good-quality but reasonable feedline like RG-213 to connect it all up. He showed that the losses in that range were low enough that for most people the cost savings over a remote tuner was a good trade-off.

    Unfortunately, it is almost impossible to run an efficient HF mobile antenna without remote matching. Whether that's an SGC coupler or a screwdriver, remote matching is the way to go for mobile ops (the OP's original problem).
     
    W0VRA likes this.
  9. W0VRA

    W0VRA Ham Member QRZ Page

    I have enough wire to build a couple of 40m λ/2 dipoles. I'm thinking about also putting together a OCFD for 40m (or 80) as well. I'll work 40m first since there seems to be plenty of traffic and I have a 40m rig and a HF general coverage rig. I plan on working digital and phone, so the antenna would have to work across 7.000-7.300 MHz. I could split the difference and tune the dipole to 7.150.

    I've got ferrites to build a 4:1 current balun for the OCFD.

    I've also got a noise source bridge that I should be able to use for antenna analysis with a couple of SDRs that I have.

    Sounds like I need to work!
     
  10. K1FBI

    K1FBI Premium Subscriber QRZ Page

    Screenshot_20180416-113657.png
     

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